Situations matter a great deal for fantasy purposes IMO, RBs especially. They usually tend to disappear when teams abandon the run game trying to catch up by passing. So if a RB gets 100+ yards in a blowout game, I'd like to know if he got it when the game was close or if he got it when the other defense didn't care. It would be very easy to get the misconception that a RB was a big stud against a stout defense for getting 100+ yards if one was ignorant of how that RB got his yards. And not very helpful when using that game to determine if that RB should be used in a future matchup.
WRs are a different matter though. They're still involved with the game when their team is trying to catch up through the air. If they're targetted during garbage time, it's a good assumption that that WR is targetted in any situation.
Basically, a team's defense can clue you in on how that team's offensive players may fare throughout the season. Bad teams like SF and Arizona will be behind and throw a lot, usually abandoning the running game.
TEs are another position where it's helpful to know their situation. Take Witten for example. He was a horrible fantasy player when Bledsoe was the QB and he was needed to help block most of the game. Now that Romo is the QB, we know that he's freed up to run routes and catch passes.